1. Field of the Invention
The present invention relates to a photo-electronic integrated circuit device having a photo-emission/detection element and an electric element and used in a photo-detector of optical fiber communication.
2. Related Background Art
An example of a prior art photo-electronic integrated circuit device (OEIC) is shown in FIG. 1. The OEIC is monolithically formed on one semiconductor substrate and comprises a photo-diode PD, resistor RG, R1 and R2, and high electron mobility transistor (HEMT) Q1 and Q2. In the circuit of FIG. 1, the photo-diode PD and the resistor RG constitute a photo-detector stage, the resistor R1 and the transistor Q1 constitute an amplifier stage, and the transistor Q2 and the resistor R2 constitute a source follower stage.
In the prior art OEIC, since a light signal is very small in the photo-detector stage which converts the light signal to an electrical signal, the amplifier stage which amplifies the output signal of the photo-diode PD has a very high sensitivity. Accordingly, when an electrical noise signal is externally applied, a final stage of the amplifier stage is significantly affected by the noise.
The transistor Q1 and the load resistor R1 in the amplifier stage constitute an inverting amplifier 20. In order for the inverting amplifier 20 to have a unity gain, it is designed such that a product of a conductance of the transistor Q1 and the load resistor R1 is unity. When such a design is applied to a field effect transistor comprising an AlInAs layer and a GaInAs layer on an InP substrate, a voltage of higher than 4 volts is applied across the source and the drain of the transistor Q1 assuming that a power supply voltage VD1 is 5 volts. Since this voltage essentially corresponds to a breakdown voltage between the source and the drain of the transistor Q1, it causes a problem of likelihood of the breakdown of the transistor.